1. Field of the Invention:
The present invention generally relates to the field of ion cyclotron resonance (ICR) spectroscopy and, more particularly, to a new ion and electron detector for use in an ICR spectrometer.
2. Description of the Prior Art:
In an ICR spectrometer the analyzer cell which is also referred to as the resonance cell is typically rectangular in shape and consists of four plates. These include a base plate which is parallel to and spaced apart along a first axis from a top plate and two parallel side plates which are spaced apart from each other along a second axis, which is orthogonal to the first axis. The base and top plates are often referred to as the drift or resonance plates and the two side plates as the trapping plates. A magnetic field is applied in the direction of the second axis, i.e., in the direction along which the trapping plates are spaced apart. A dc potential is applied to each of the trapping plates with respect to the base plate in order to establish a potential thereacross, whose function is to center any ions in the cell about the cell center. The types of ions present in the cell are detected by a detector which is connected across the resonance plates.
The detector which is most extensively used in the prior art is of the marginal oscillator type. Basically, it consists of a tuned circuit in which the resonance plates define a capacitive element of simply a capacitor. The marginal oscillator detector is tuned to a fixed frequency in the radio frequency (rf) range, and uses a feedback resistor to provide oscillation at a constant rf amplitude.
The magnetic field causes the ions in the cell to undergo a cyclotron motion or oscillation in a plane perpendicular to the magnetic field. The frequency of the cyclotron motion, hereinafter also referred to as the cyclotron frequency oscillation, is dependent on the mass of the ions in the cell and the magnetic field. When the ion's cyclotron frequency equals that of the detector's fixed frequency, the Q of the detector's tuned circuit drops sharply, which causes a sharp drop in the detector's output. The mass of the ions present in the cell is identified from the rf frequency and the amplitude of the magnetic field when such drops occurs. Such cells and the marginal oscillator detector are extensively described in the literature and in U.S. Patents including U.S. Pat. Nos. 3,446,957, 3,475,605, 3,505,516, and 3,511,986. See for example, U.S. Pat. No. 3,475,605, column 2, line 66 in which the oscillator detector is referred to as the marginal oscillator.
There are several significant disadvantages in the prior art arrangements. The cyclotron frequency of low mass ions with M/e = 1-10, except at very weak magnetic fields below about 3,000 gauss (G), is on the order of several megahertz (MHz). At frequencies of several MHz the rf level of the marginal oscillator detector is too high for most ICR experiments. A high rf level excites ion cyclotron motion of large amplitude, causing many ions to strike the resonance plates and thereby become neutralized. This causes undesired loss of ions from the cell. It is for this reason that the rf frequency provided in the prior art by the marginal oscillator detector is limited to be below about 1MHz and thereby limit the rf level to be sufficiently low for the ICR experiments. The desired rf level is on the order of 20mV and less for most experiments. Since the cyclotron frequency of low mass ions, except at weak magnetic fields, is generally above 1MHz, these ions cannot be detected easily with the marginal oscillator detector at typical magnetic fields between 3,000 and 12,000 G. For the foregoing reasons, the marginal oscillator detector is not very useful for the detector of ions in the ICR spectrometer where the cyclotron frequencies are on the order of several Megahertz, e.g., 2-15MHz. The marginal oscillator detector cannot produce such rf frequencies at sufficiently low rf levels.
In addition to the above limitations the marginal oscillator detector is completely useless to detect electrons, whose cyclotron frequency is in the Gigahertz range. Thus, a need exists for a new type detector for use with an ICR resonance cell to detect ions at high cyclotron frequencies, e.g., 2-15MHz and a need exists for an arrangement to detect the presence of electrons in an ICR cell.